Anyone ever use a Schmidt hammer? How? Can they be used on 365+ day old concrete?

I'm considering investing in one but don't know how they work.

I'm starting to think a lot of my headaches are from either
a) soft crete - concrete that has a surface problem that falls apart after I have the floor down
b) hard crete - hard troweled concrete that inhibits a normal speed of moisture movement out of the slab.

Oh yeah, the rebound hammer! It is designed to tell you the compressive strength of the concrete. They are good to get a number, but I was pretty disappointed to see Mr. Craig didn't use one on a project we did...

He just scraped the concrete surface with a screwdriver. If you can scratch the surface semi-easily, something is wrong.

If the slab doesn't completely absorb a drop of water in one minute, it's not porous enough.

Those are the tests I use 100% of the time. If I need to document something I will use a rebound hammer and an elcometer to get further numbers.